The continuous casting of steel product is a technology that has been developed within the last 30 years. In continuous casting, a molten charge of steel is prepared in a ladle vessel that is moved into the proximity of the continuous casting device. Slag is inherently present in the steel-making process. This slag floats to the top of the molten steel in the ladle due to it's lower density. The slag generally comprises the various iron oxides: ferrous oxide (FeO); ferric oxide (Fe.sub.2 O.sub.3); and ferroferric oxide (Fe.sub.3 O.sub.4). When the ladle is positioned, the molten steel is drained from the bottom of the ladle through a channel called a ladle shroud into a tundish. The tundish acts as both a reservoir and a manifold for feeding the continuous casting molds.
Molten steel enters the top of the tundish and is fed out of the bottom of the tundish through one or more pipes into molds where a slab of steel is formed and continuously pulled away for further processing. As successive ladles of molten steel are brought into position and then ultimately removed, the tundish serves as the reservoir that allows the molding process to operate continuously.
The economic advantages of continuous casting can be realized only when successive ladles of molten steel are charged to the process without introducing any slag into the product but also without leaving steel in the ladle. Therefore, it is important to detect the presence of slag in the ladle shroud and, once slag is detected, to close the gate valve feeding molten steel from the ladle into the ladle shroud so that the ladle may be replaced.
Molten steel and steel slag exhibit dramatically different electromagnetic properties, so past attempts to detect the presence of slag have relied upon this and used an electromagnetic coil. The use of a coil has unfortunately not provided the desired accuracy.
Another method of detecting the presence of slag is to allow a certain amount of slag to enter the tundish, where the eruption of the slag from the bottom of the ladle shroud can be visually observed due to what is known as the "volcano effect." As with any visual technique, however, it is subject to the subjective judgment of a human viewer, also the attention of a human viewer, both of which are unfortunately unreliable.